Introduction
Grassland is a central component of the terrestrial ecosystem, playing a prominent role in maintaining the world’s ecological balance (Gravuer et al., 2019; Prommer et al., 2020). Grasslands across arid and semi-arid regions are currently undergoing widespread shrub encroachment, which were described as the rise in dominance and abundance of shrubby plants in grasslands (Stevens et al., 2017; Aguirre et al., 2021). These ecosystem transformations may be caused by combinations of climatic variation and anthropogenic processes, including large increases in precipitation, air temperature, nitrogen deposition, as well as grazing disturbances (Formica et al., 2014; Pistón et al. 2016). Shrub encroaching into grasslands often caused abrupt variations in biodiversity, vegetation productivity, hydrological properties, carbon dynamics, and soil nutrients (Liu et al., 2021; Broadbent et al., 2022). These changes are closely linked with alterations in not only the overall but also above- and belowground ecosystem functioning (Eldridge et al., 2011; Valencia et al., 2015).
Alpine ecosystem on the Tibetan Plateau has undergone rapid climate changes and intensity anthropogenic activities in recent decades (Kuang et al. 2016), which are also associated with substantial structural and functional variations of alpine ecosystems, particularly the prevalently ecological process known as shrub encroachment (Wu et al., 2021; Zhao et al., 2023). In general, compared with herbaceous species, shrub species with greater aboveground and root biomass can provide more organic matter input soils (Zhou et al. 2017; Cui et al., 2023) and enhance the accumulations of soil carbon and nitrogen (Li et al., 2019; Zhao et al., 2023). Shrub encroached into alpine grassland ecosystems can increase biodiversity, soil fertility, soil infiltration processes, ultimately attributing to strengthen ecosystem functioning (Butterfield et al., 2016; Cai et al., 2020). Nevertheless, neutral or even negative ecosystem individual and multiple functioning response to shrub encroachment also existed in alpine ecosystems (Zhang et al., 2022; Yang et al., 2023). Thus, the ecological impacts of shrub encroachment are expected to affect biodiversity and even lead to the shifts of ecological functioning in alpine ecosystems, which depend on a variety of factors, including shrub traits, edaphic condition, and local climate (Brandt et al., 2013; Collins et al., 2020).
In semi-arid alpine grasslands, increasing evidence suggesting that shrub encroachment can have the potential positive effects on the ecosystem functions, including plant diversity, soil hydrological, and soil fertility (Noumi et al., 2016; Wang et al., 2023). Nevertheless, the degree by which shrub encroachment caused variations in the ecosystem functioning could vary with the traits of encroaching shrubs (e.g., Leguminosae or Non-Leguminosae species). In semi-arid regions, Leguminosae shrubs can overcome drought stress and nutrients constraints to promote species richness, vegetation productivity, and carbon accumulation (Saixiyala et al., 2017; Ale et al., 2023). However, the encroachment of Leguminosae shrubs was differently associated with the ecosystem functioning than that of none-leguminous shrubs (Li et al., 2016; Zhao et al., 2023). Nevertheless, their wider positive impacts on belowground ecosystem functioning, and the underlying mechanisms and the changes magnitude of the ecosystem functioning of the semi-arid alpine steppes induced by shrub encroaching, remain poorly understood.
Much less attention has been given to recent widespread encroachments of shrubs in semi-arid alpine ecosystems, such as leguminous shrub (Caragana spinifera ) and none-leguminous shrub (Dasiphora fruticosa ). These encroaching shrubs was expected to provide favorable habitat for herbaceous plants due to increased shrubs abundance and sprawling canopy that provide increased ultraviolet radiation protection and buffering of extreme soil temperatures and drought stress, as well as an increase in soil nutrients availability (Cui et al., 2023; Zhao et al., 2023), which are associate with strengthening ecosystem functioning. These dwarf shrubs may be associated with different above- and belowground effects, including increased vegetation productivity and increased soil nutrient contents. We tested two hypotheses. First, we hypothesized that encroachment of both Leguminosae and non-Leguminosae shrubs had beneficial effects on the ecosystem functioning of the semi-arid alpine steppes, which was associated with increased vegetation productivity and higher soil nutrient availability. Second, we hypothesized that above- and belowground ecosystem functioning of the semi-arid alpine steppes responded differently to shrub encroachment, with the belowground ecosystem functions increased greatly than the aboveground ecosystem functions.
Here two shrub encroached alpine steppe sites of the central Tibetan Plateau were surveyed, with one site encroached by a leguminous shrub (Caragana spinifera ) and the other site encroached by an none-leguminous shrub (Dasiphora fruticosa ) . The changes in the individual and multiple ecological functions of the semi-arid alpine steppes following the encroachments of leguminous shrub and none-leguminous shrub were assessed. The aims of our study were to (1) quantify the impacts of encroached by leguminous shrub and none-leguminous shrubs on above- and belowground ecosystem functioning of semi-arid alpine steppes, and (2) identify the underlying mechanisms of encroaching shrubs induced variations in ecosystem functioning in the semi-arid alpine steppes.